Recently, a document reading device for use in scanners, multifunction copying machines or the like, has become remarkably faster in reading speed per line. In a signal-processing unit to be applied to this document-reading device, it is necessary to divide one line into a plurality of blocks to make a parallel processing, as well as to process at higher speed each of the blocks having been divided. Moreover, a circuit for processing analog data read out individually in each of the plurality of blocks needs to operate at higher speed as a matter of course. On the other hand, conforming to document-reading devices being downsized, a signal-processing unit also tends to be downsized by, e.g., being formed into one chip.
Basically, each block in a signal-processing unit is connected through a switched capacitor of which ON/OFF is controlled at drive clock. Fetching data from each block, and transferring data to the subsequent block are processed by adjustment of a timing of drive clock so as to prevent interference between signals processed by each block.
In the conventional signal-processing unit, however, a problem exists in that a state in which outputs of adjacent signals are affected each other depending on timing of switching of each circuit block occurs when the signal-processing unit is driven at high speed by a high-speed clock, or that layout and wiring run of each circuit become hard as circuits are downsized, resulting in the interference of wiring with each other. Thus, the interference between signal data with each other, so-called cross talk, occurs between each of the circuits. Therefore, a further problem exists in that data after having been fetched in the signal-processing unit are changed or deteriorated as compared with data before being fetched in the signal-processing unit, and that correlation between data before signal processing and data after signal processing cannot be obtained. Furthermore, as document reading devices are downsized and operated at high speed, there arise situations in which wiring between plural lines of signals to be parallel-processed comes to be proximate, thus the cross talk between wirings may occur in early stage of the signal-processing unit.
As measures against these situations, to solve the problems as mentioned above, it has been attempted that the circuit layout in an internal part of a signal-processing unit, or switching timing between blocks are optimized; or wiring in the document reading device is designed to prevent the lines from being proximate to each other to a minimum. In actual, however, there remains the cross talk that is not eliminated even by those measures. For example, in the case of reading documents with the use of such a signal-processing unit for processing read data of the document reading device, the so-called “ghost phenomenon” in which a certain part of output data is seen in the other part like a thin shadow occurs, resulting in deterioration of picture quality.
The invention was made to solve the above-mentioned problems, and has an object of providing a signal-processing unit making it possible to effectively process the cross talk between signal data that occurs when a plurality of signal lines are processed in parallel.